1. Field of the Invention
This invention relates to windings for the stators of electric machines such as motors and generators of the axial gap or radial gap type. Both brushless and brushed type DC motors and generators are improved by having plural coils assembled arcuately adjacent one another with the active sector of the radial sections of the coils in an essentially coplanar configuration and with the radially active sector having tapered conductors that increase in width in the radial direction.
2. Description of Related Art
Electric motors and generators of the axial gap type are well known. They employ motors that deliver axially directed magnetic fields to stationary radially directed electrically conductive coils of wire. It is common to have rotors with permanent magnets of alternate polarity with the magnets of opposite or opposing fields attracting each other, concentrating the magnetic field across a stator sandwiched in between the permanent magnet rotors. In the motor mode, electric current is passed through the stator windings. Currents flowing through the radially positioned conductors intersect the axial field created by the magnets to produce a torque that rotates the motor rotor. In the generator mode the magnets are rotated by an external prime mover. When this happens, the rotating magnet's axial field interacts with the radial conductor coils to induce a current flow in and voltage across the conductor coils. FIG. 1 illustrates a prior art configuration showing the direction of current, magnetic field and resulting force. FIG. 2 illustrates a common prior art phase coil arrangement. The phase coil is a continuous conductor wound flat in a closed loop form. It generally consists of "active sectors" or areas (i.e., those portions of the coil that extend radially out from and perpendicular to the axis of rotation of the rotor and that cross the magnetic field created by the magnets) and "inactive sectors" or areas (i.e. those end windings or portions of the coil that are outside the magnetic field which connect the active sectors). The current in the active portion of the coils flows radially outwardly on one side and radially inwardly on the other or opposite side of the coil.
The basic theories pertaining to, and several illustrations and versions of, axial gap machines are set forth in an article by P. Campbell in Proceedings of the Institute of Electrical Engineers, Vol. 121, No. 12, December 1974, pages 1489-1494, and an article titled "Principles of a Permanent-Magnet Axial-field d.c. Machine," by P. Profumo, Z. Zhang and A. Tenconi in IEEE Transactions on Industrial Electronics, Vol. 44, No. 1, February 1997, pages 29-45; which are incorporated herewith by reference.
FIG. 1 illustrates the interaction of the magnetic fields and currents under consideration in the prior art axial gap motor 10. Two rotating permanent magnet disks 1, 2 sandwich a stator ring 3 separated by two air gaps 4, 5 as they rotate about an axis 7 of a rotor axle 12. The rotating magnet rings 1, 2, have alternately poled flat permanent magnets, typically of sector shape, which produce an axially directed magnetic field 6 across the air gaps. The rotating magnet disks are supported by iron rings 13, 14. The magnetic gap is the distance between the two facing surfaces of the rotating permanent magnets. The stator ring 3, supported by a housing 15, contains sets of phase coils. Radially directed current 8 in the active part of the coil conductors interacts with the axially directed magnetic field 6 to produce a tangentially directed force 9 which creates the torque that drives the motor.
FIG. 2 illustrates a typical prior art arrangement of sector-shaped coils 20. A rotor rotates about an axis of rotation 7 in an opening 25 provided in the stator. The rotor has sector-shaped permanent magnets on it with the inner radial extent of the magnets 28 and the outer radial extent of the magnets 29 indicating where the magnets rotate past the coils 20 of the stator. The north poles and south poles of adjacent magnets alternate around the rotor. Three phase coils A,B,C are used together with three phase electrical machines. Although there is continuous relative motion between the magnets and coils in the region occupied by the permanent magnet pole pairs, N-pole,S-pole, the coils are shown directly over the pole pairs. The coils each have an inner inactive section 27 and an outer inactive section 26, referred to as end windings. Radial sections 22a, 22b extend between the inner and outer sections of each coil. An open area 24 is formed between the coil sections. Three of the radial sections 22a of the coils are shown crossing the magnetic field created by the north pole magnets while their opposite radial sections 22b are crossing the magnetic field created by the south pole magnets. The active sector of the coil radial sections is that portion of the radial sections 22a, 22b that extends between the magnet inner radial extent 28 and the magnet outer radial extent 29.
When using uniform cross-section conductors these "empty areas" 24 equal 1.sub.r /(2r.sub.i) fraction of the conductor area where 1.sub.r =active length of conductor 22 under the rotating magnets and r.sub.i =inner radius 27 of the coil. For a typical design with 1.sub.r =r.sub.i, this implies that 50% of the space available for conductors in the stator is not used to produce torque. This ratio of "empty area" to "conductor area" is called the "spoke fill factor." This loss of useful area is unique to axial motors. It results in a loss in torque that is in addition to the conventional "shape fill factor", that defines the loss of area due to the shape of the wire, and the "insulation fill factor", that defines the loss of area due to the insulation coating on the bare copper wire. The combination of "shape fill factor" and "insulation fill factor" is called the "copper fill factor." As an example, round wires have a "copper fill factor" of 0.7. This loss coupled with a "spoke fill factor" of 50% yields a total fill factor of (0.7)(0.5)=0.35 or 35% for round wires. The situation is worse if Litz wires are used. Litz wires have typical "copper fill factors" of 0.4 to 0.55. This coupled with the "spoke fill factor" of 50% can yield a still lower total fill factor of (0.4)(0.5)=0.20 or 20%. This implies that only 20% of the available area is used to produce torque. As a result, the torque produced is significantly lower than what is possible when the entire available geometric area is used to produce torque.
In order to increase torque, many axial gap machines use three phase windings. The coils are arranged to form wave windings and phase coil windings. Typically three phase coils are positioned under each permanent magnet pole. Two of these coils are energized at any time by connecting them in a wye or a delta mode, thus having conductors crossing two-thirds of the pole pitch for torque generation. Pole pitch is the arc length from the center of a north pole to the center of a south pole measured along the mean radius of the permanent magnets.
Many of the problems solved by the present invention have been addressed by the prior art. Ban et al, U.S. Pat. No. 4,107,587, issued Aug. 15, 1978, address the problem created by increasing the gap between magnets in a three-phase motor. The coil(s) of one phase are shifted by 180.degree. from the next phase and not superimposed. A. Tong, U.S. Pat. No. 4,916,345, issued Apr. 10, 1990, uses spaced tabs to connect adjacent windings. Takahashi et al, U.S. Pat. No. 4,551,645, issued Nov. 5, 1985, use loop-like windings on a stator in a disc type motor using permanent magnets with various electrical arrangements. Windings that utilized the shape of a wave having three conducting sides are well known and described in the literature. U.S. Pat. No. 4,319,152 to A. van Gills, issued Mar. 9, 1982, discloses laminated coil windings formed by multiple layers of thin electrical conductive material shaped so that current flowing through them generates a magnetic pole at three of four cross-sectional sides. The laminated coil is formed of layers of patterns arranged to provide a magnetic pole at all four sides.
In such prior art three-phase motors, the flat armature coils, superimposed in three layers, result in an increased axial thickness of the stator. This three-fold increase in thickness of the stator greatly increases the gap between the permanent magnets. This configuration weakens the strength of the magnetic field which passes between the magnets across the coils. As a result the torque is greatly reduced. Reduced field strength requires significantly larger and/or more powerful permanent magnets or increases the current required to produce a given torque, which in turn reduces the efficiency because of increased ohmic losses.